1. Field of the Invention
The present invention relates to light-emitting elements using an organic electroluminescence (EL) phenomenon (hereinafter such light-emitting elements are also referred to as organic EL elements).
2. Description of the Related Art
An organic EL element has been actively researched and developed. In a fundamental structure of the organic EL element, a light-emitting layer containing a light-emitting material is interposed between a pair of electrodes. The organic EL element has attracted attention as a next-generation flat panel display element owing to characteristics such as feasibility of being thinner and lighter, high speed response to input signals, and capability of direct current low voltage driving. In addition, a display using such a light-emitting element has a feature that it is excellent in contrast and image quality, and has a wide viewing angle. Further, being a plane light source, the organic EL element has been attempted to be applied as a light source such as a backlight of a liquid crystal display and a lighting device.
The emission mechanism of the organic EL element is of a carrier-injection type. That is, by application of voltage with a light-emitting layer interposed between electrodes, electrons and holes injected from the electrodes are recombined to make a light-emitting material excited, and light is emitted when the excited state relaxes to the ground state. There can be two types of the excited states: a singlet excited state (S*) and a triplet excited state (T*). The statistical generation ratio of the excited states in a light-emitting element is considered to be S*:T*=1:3.
In general, the ground state of a light-emitting organic compound is a singlet state. Therefore, light emission from the singlet excited state (S*) is referred to as fluorescence because it is caused by electron transition between the same spin multiplicities. On the other hand, light emission from the triplet excited state (T*) is referred to as phosphorescence where electron transition occurs between different spin multiplicities. Here, in a compound exhibiting fluorescence (hereinafter referred to as fluorescent compound), in general, phosphorescence is not observed at room temperature, and only fluorescence is observed. Accordingly, the internal quantum efficiency (the ratio of generated photons to injected carriers) in a light-emitting element containing a fluorescent compound is assumed to have a theoretical limit of 25% based on S*:T*=1:3.
On the other hand, when a compound exhibiting phosphorescence (hereinafter referred to as a phosphorescent compound) is used, the internal quantum efficiency can be theoretically increased to 100%. That is, higher emission efficiency can be obtained than using a fluorescent compound. For these reasons, a light-emitting element containing a phosphorescent compound has been actively developed in recent years in order to achieve a high-efficiency light-emitting element. As the phosphorescent compound, an organometallic complex that has iridium or the like as a central metal has particularly attracted attention because such an organometallic complex has a high phosphorescence quantum yield. For example, an organometallic complex that has iridium as a central metal is disclosed as a phosphorescent material in Patent Document 1.
When a light-emitting layer of a light-emitting element is formed using the above-described phosphorescent compound, in order to suppress concentration quenching or quenching due to triplet-triplet annihilation of the phosphorescent compound, the light-emitting layer is often formed such that the phosphorescent compound is dispersed in a matrix of another compound. Here, the compound serving as the matrix is called a host material, and the compound dispersed in the matrix, such as a phosphorescent compound, is called a guest material.